More than 30 years ago, the Mariner 10 spacecraft shot past Mercury three times, beaming back most of what we know today about this enigmatic world.

Angela Cesere
Graphic by Gervis Menzies

“We got sort of a tantalizing glimpse,” said Patrick Koehn, assistant research scientist at the University’s Space Physics Research Laboratory, of the first mission to Mercury.

Now scientists are venturing back to Mercury with NASA’s MESSENGER spacecraft, which was launched last August, and will begin a yearlong orbit of the planet in 2011. MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, will be the first spacecraft ever to orbit Mercury and study the planet in detail. This time, scientists hope to finally crack some of Mercury’s mysteries through new technologies, particularly the Fast Imaging Plasma Spectrometer developed by University researchers.

Mercury, the Earth’s neighbor closest to the sun, basks in sunlight 11 times greater than that on Earth, scorching the planet with temperatures reaching 840 degrees Fahrenheit. Yet observations of Mercury show that it is home to ice. Also mysterious is Mercury’s extremely high density, unmatched by Venus, Earth and Mars. 65 percent of Mercury’s mass is in its metallic core – a core twice the size of Earth’s. In addition, planetary magnetic fields have only ever been explained as the result of an active liquid interior. With its solid core, Mercury should not have a magnetic field – but it does.

“Mercury is one of the least explored planets. It is extreme in many ways,” said Thomas Zurbuchen, Project Director for the construction of FIPS.

“From that orbit, it will explore the planet and all its properties: the interior, the surface, the environment of the planet.” Zurbuchen said. “We expect to learn where the planet came from, and how it is part of the solar system.”

Zurbuchen and Koehn, with the help of SPRL faculty and students, and backed by a $3 million grant from NASA, built the small, three-pound FIPS, a first-of-its-kind instrument that will take the first ever measurements of the activity of charged particles in Mercury’s environment.

As MESSENGER cruises through Mercury’s magnetosphere – the region surrounding the planet that contains charged particles controlling Mercury’s magnetic field – FIPS will function like a camera to detect different types of these particles and their velocities. It is the most compact instrument of this type ever developed, and, unlike previous narrow-lensed models, provides scientist with a wider, “fish eye” view.

“It was a real innovation,” said Zurbuchen. “We’re going to go to a place with an instrument that has never been there. We’re going to learn new science.”

Data gathered by FIPS will provide new insight into how the solar wind interacts with planetary magnetic fields. By having an unsubstantial atmosphere and a magnetosphere, Koehn said, Mercury is perfect for studying interaction between the solar wind and planetary magnetic fields.

While both Earth and Mercury each have a magnetosphere, Earth’s atmosphere is much thicker, while Mercury’s is very light, similar to that of the Moon’s. As Earth’s magnetosphere is hit by the solar wind, it responds by interacting with Earth’s underlying atmosphere.

Mercury, having virtually no atmosphere, gives scientists the opportunity to study the interaction between the solar wind and a magnetosphere without interference from an atmosphere. In effect, Mercury allows for a controlled experiment that will test scientists’ general understanding of how the solar wind interacts with planets.

“With the Mercury system we have one less variable,” Koehn said. “You can think of it as a planetary laboratory.”

But the overall goals of MESSENGER are even more far-reaching.

According to NASA’s website, surface composition measurements will reveal how Mercury became the dense planet it is today. If the surface is similar to other terrestrial planets, then Mercury’s density was caused simply by an accumulation of dense particles as the solar system formed.

However, other compositions might suggest more violent pasts. For instance, if MESSENGER observes a surface low in elements that evaporate easily like sodium and potassium, it would suggest that the early sun vaporized the outer surface of Mercury, leaving it the dense cinder it is today. If the surface is low in elements that would have made up Mercury’s primordial crust like silicon, aluminum and oxygen, it would suggest that this crust was stripped of Mercury by giant impacts soon after the planet formed.

Mercury’s magnetic field will also be explored. For the most part, planetary magnetic fields are generated by an active liquid interior, but observations of Mercury’s rotation indicate that it must have a solid iron core, Zurbuchen said.

“So how can it have a magnetic field? All these things we use to explain the Earth’s magnetic field, or the sun’s magnetic field, all these theories can’t apply.” Zurbuchen said.

Before MESSENGER begins to orbit Mercury, it will have traveled 4.9 billion miles by conducting five deep space maneuvers, acclerating the speed of the spacecraft in short bursts so that it can travel along the gravitational pull of the planets to jump from the Earth to Venus and then on to its final destination — Mercury.

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